Technical Field
The present disclosure relates to an encapsulated device of semiconductor material with reduced sensitivity to thermo-mechanical stresses. In particular, the following description regards a MEMS (Micro-Electro-Mechanical System) device of an inertial type, such as an accelerometer or a gyroscope of a capacitive type, without the disclosure being limited thereto.
Description of the Related Art
As is known, semiconductor devices, such as MEMS devices, are generally encapsulated in packages to enable protection and handling thereof. Hereinafter, reference is made to packages enabling surface mounting, even though the disclosure is not limited thereto.
Currently, the type of surface-mount package that is most widely used for MEMS sensors is the so-called LGA (Land-Grid Array) package, which has a square grid of contacts on the bottom side of the package. FIGS. 1 and 2 illustrate an example of package of an LGA type in cross-section and in perspective view from the back, respectively.
In particular, FIG. 1 shows an example of an encapsulated MEMS device 1, comprising a base 2, a cap 3, a first chip 4 bonded to the base 2, and a second chip 5 bonded to the first chip. The base 2 may be formed by a support of different material, for example of fiberglass or ceramic, and the cap may be metal, polymeric, or ceramic material. The first chip 4 may be a MEMS component, for example of a capacitive type and including sensing structures of an inertial type, such as an accelerometer or a gyroscope, and the second chip 5 may be an integrated circuit, such as an ASIC (Application-Specific Integrated Circuit), including signal-processing circuits.
Generally, the second chip 5 is electrically coupled to the first chip 4 so as to receive the measuring signals supplied by the latter and supply on the outside the values of the measured quantities and/or of quantities correlated thereto.
As an alternative to the above, the package may be obtained with the full-molded technique, and the first and second chips 4, 5 may be surrounded by an encapsulation mass that completely envelops them and fills the volume of the package.
FIG. 2 shows the arrangement of the rear contacts, designated by 10 and arranged peripherally with respect to the base 2. The contacts 10 are made of metal material, such as copper, and are connected to the second chip 5 via conductive bonding wires 7 and through vias 8 (FIG. 1). Conductive wires 9 connect the chips 4, 5 together.
FIG. 3 shows a schematic top plan view of an example of the structure of an MEMS component 5 forming an accelerometer or an inertial gyroscope. The MEMS component 5 comprises a suspended mass 15 arranged over a substrate (not visible in FIG. 3) and supported by a fixed region 16 via springs 17. The fixed region 16 extends all around the suspended mass 15 and is fixed with respect to the substrate. Fixed electrodes 18 extend from the fixed region 16 to the suspended mass 15 and are combfingered to mobile electrodes 19, which extend from the suspended mass 15 to the fixed region 16.
Contact pads 22 on the fixed region 16 are coupled to the terminals 10 by the bonding wires of FIGS. 1 and 2.
The shown package structure is sensitive to thermo-mechanical stresses, for example generated by temperature jumps, humidity, aging, environmental conditions and other mechanical stresses that cause bending or other deformation of the base 2. For instance, since the materials of the first chip 4 and of the base 2 are different, and thus have different thermal expansion coefficients, an exposure to temperature gradients may cause deformations or warpage of the package. These deformations may cause changes of distance between the fixed and mobile electrodes 18, 19, thereby affecting the output parameters of the signals generated by the first chip 4, jeopardizing the accuracy of the measurements, and determining operating uncertainties.
In this connection, reference may be made to FIGS. 4A, 4B, and 5A, 5B, which show the effects on a MEMS device of a stress due to a temperature variation ΔT>0 and the effects of a stress due to a temperature variation ΔT<0, respectively.
As may be noted in FIGS. 4A, 5A, an increase in temperature may cause bending of the substrate (here designated by 25) upwards (the convexity facing the electrodes 18, 19), which in turn causes an increase of the rest distance g0. Instead, a reduction of temperature (FIGS. 4B, 5B) may cause bending of the substrate 25 downwards (the concavity facing the electrodes 18, 19) that causes a reduction of the rest distance g0.
For instance, when the chip 4 is an accelerometer, the modification of the differential distance may modify the value of the d.c. signals (the so-called “0-g level drift”) and the expected sensitivity drift. When the chip 4 is a gyroscope, the modification of distance between the electrodes may affect the quality factor, the resonance frequency, and the quadrature, bringing about a zero-level drift and, also in this case, the expected sensitivity drift.
To eliminate or at least reduce the effect of mechanical stresses on the output parameters of the MEMS device, various solutions have been proposed both of an architectural type and of a structural type.
For instance, U.S. Pat. No. 8,434,364 proposes an optimization of the anchoring position for reducing the deviation of the parameter values of the output signals in the presence of warpage of the substrate of the chip.
Other solutions envisage the use of packages that employ low-stress materials and/or materials having similar stress characteristics. For instance, FIG. 6 shows an encapsulated device 30 wherein the package is formed by a ceramic body 31 and a lid 32, also generally made of ceramic material. The ceramic body 31 has a cavity 33 housing first and second chips 34, 35. For instance, the first chip 34 is a MEMS sensor, and is bonded on the bottom of the ceramic body 31, within the cavity 33, via a first adhesive layer 371, for example a continuous DAF (Die-Attach Film) layer, used in the semiconductor industry. For instance, the first adhesive layer 371 may be a laminated epoxy resin. The second chip 35 is, for example, formed by an ASIC and is bonded on top of the first chip 34 via a second adhesive layer 372, for example a DAF layer. Electrical connections (not shown) couple the chips 34 and 35 to terminals 36 formed in the ceramic body 31.
FIG. 6 further schematically shows fixed electrodes 38 rigid with a substrate 39 of the first chip 34 and a mobile electrode 40 facing the fixed electrodes 38.
The ceramic material used has the advantage of having a thermal expansion coefficient similar to silicon that forms the chips 34, 35, thus reducing the deformations due to thermal effects, and attenuating mechanical stresses coming from outside, but is not sufficient to reduce the reliability of the variations of parameters in all the other cases.
It follows that current solutions are not always sufficient to eliminate the undesirable effects.